South Korea resumes anti-North broadcasts after H-bomb test

South Korean soldiers adjust equipment at a broadcasting room as they prepare propaganda broadcasts near the border area between South Korea and North Korea in Yeoncheon, northeast of Seoul, on January 8, 2016South Korean soldiers adjust equipment at a broadcasting room as they prepare propaganda broadcasts near the border area between South Korea and North Korea in Yeoncheon, northeast of Seoul, on January 8, 2016Cameramen film video footage of North Korea's border county of Kaepoong (background) from a South Korean observation post in Paju near the Demilitarized Zone dividing two Koreas on January 8, 2016.

South Korea retaliated for North Korea's nuclear test with broadcasts of anti-Pyongyang propaganda across the rival's border.

Today is believed to be the birthday of North Korean leader Kim Jong Un, who is thought to be in his early 30s, and the broadcasts will draw a furious response from North Korea, which considers them an act of psychological warfare.

South Korean soldiers adjust equipment at a broadcasting room as they prepare propaganda broadcasts near the border area between South Korea and North Korea in Yeoncheon, northeast of Seoul, on January 8, 2016

Pyongyang is extremely sensitive to any outside criticism of his authoritarian leadership, and its reaction could be especially harsh because of the high emotions surrounding his likely birthday.

When South Korea briefly resumed propaganda broadcasts in August after an 11-year break, the two Koreas exchanged artillery fire, followed by threats of war.

South Korea's Yonhap news agency reported that frontline troops, near 11 sites where propaganda loudspeakers started blaring messages, were on highest alert.

Yonhap said Seoul had deployed missiles, artillery and other weapons systems near the border to swiftly deal with any possible North Korean provocation.

Cameramen film video footage of North Korea's border county of Kaepoong (background) from a South Korean observation post in Paju near the Demilitarized Zone dividing two Koreas on January 8, 2016.

The broadcasts include popular Korean pop songs, world news and weather forecasts as well as criticism of the North's nuclear test, its troubled economy and dire human rights conditions.

Included are songs by a young female singer, IU, which might be aimed at North Korean soldiers deployed near the border.

August's broadcasts, which began after Seoul blamed Pyongyang for land mine explosions that maimed two South Korean soldiers, stopped only after the rivals agreed on a set of measures aimed at easing anger.

It may take weeks or longer to confirm or refute the North's claim that it successfully tested a hydrogen bomb, which would mark a major and unanticipated advance for its still-limited nuclear arsenal.

Even a test of an atomic bomb, a less sophisticated and less powerful weapon, would push its scientists and engineers closer to their goal of building a nuclear warhead small enough to place on a missile that can reach the US mainland.

Later today, South Korea is due to announce the results of its first round of investigations of samples collected from sea operations to see if radioactive elements leaked from the North's test.

President Barack Obama has spoken to South Korean president Park Geun-hye and Japanese prime minister Shinzo Abe and reaffirmed the "unshakeable US commitment" to the security of the two Asian allies.

The UN Security Council held an emergency session and pledged to swiftly pursue new sanctions against North Korea, saying its test was a "clear violation" of previous UN resolutions.